9класс вычислить: cos240градусов cos98cos53+sin98sin93= sin56cos26-cos56sin26= :

Cos240=cos(180+60)=-cos60=-1/2

cos98*cos53+sin53*cos98=cos(98-53)=cos45=√2/2

sin56*cos56-cos56*sin26=sin(56-26)sin30=1/2

sin(8π/7)*cos(6π/7)+sin(6π/7)*cos(8π/7)=sin(8π/7+6π/7)=sin2π=0

(1+sin(π+α))/(1-cos(3π/2+α))=cos²(π+α)/(1-cos3π/2+α))
1+sin(π+α)=cos²(π+α)
1-sinα=cos²α
sin²α+cos²α-sinα-cos²α=sin²α-sinα